A renewable energy certificate (REC) is a tradable document certifying that 1,000 kilowatt-hours (kWh) of electricity has been produced from a clean, renewable source. The concept is simple: electricity cannot be stored and is generated only when needed. So when you purchase one REC, you're ensuring that 1,000 kWh of clean, renewable energy are pumped into the power grid, displacing 1,000 kWh generated by a fossil fuel-burning plant.